This invention relates in general to interferometers and specifically to Michelson type interferometers. Even more specifically, this invention relates to Michelson type double pass interferometers utilized to measure the spectral intensity distribution of an incoming ray. Michelson type interferometers split a source beam into two parts, each part traversing a different optical path and then reuniting to produce interference. One of these optical paths includes a mirror mounted on an adjustable stage, the adjustment of which allows its path length to be changed.
Relatively compact spectrometers including Michelson type interferometers which are relatively insensitive to vibration and element misalignment are known. However, the prior art optical configurations are disadvantageous in that they require the use of a very large and thick beamsplitter and a 45.degree. angle of incidence of the incident beam with respect to the beamsplitter in order to achieve proper splitting of the optical beams. It is desirable to be able to use a smaller beamsplitter and an angle of incidence less than 45.degree., and the use of such a beamsplitter as described is not possible in prior art optical configurations.